The invention relates to a capped electric lamp comprising:
a quartz glass vessel which is closed in a vacuumtight manner and which has a first and a second neck-shaped portion with respective seals in mutual opposition, through which seals respective current supply conductors extend to an electric element arranged in the lamp vessel, the latter having a filling; PA1 a lamp cap connected to the lamp vessel, PA1 which current supply conductors each comprise a metal foil which is embedded in a respective seal in a vacuumtight manner and to which a respective internal current wire connected to the electric element is connected at a first end portion and a respective external current wire issuing from the relevant seal to the exterior is connected at a second end portion.
Such a capped electric lamp is known from U.S. Pat. No. 5,320,562. The lamp may be used as a vehicle headlamp, especially if the electric element is a pair of electrodes in an ionizable filling, but it may alternatively be used for other applications, for example optical applications. The lamp has the advantage of a comparatively long life and a high light output at a comparatively low power rating of approximately 35 W. The light is generated between electrodes which are spaced apart no more than a few millimeters, for example 4.5 mm, so that the lamp has a very high luminance and the generated light can be very well concentrated into a beam by a reflector and possibly a lens. The lamp vessel has comparatively small internal dimensions of, for example, approximately 1 to 3 mm diameter in the centre between the electrodes and approximately 4.5 to 9 mm length.
The known lamp may have an outer envelope around the lamp vessel, connected thereto with narrowed portions, for example, to the neck-shaped portions of this vessel.
The lamp has a lamp cap of insulating material which comprises electric contacts connected to respective current supply conductors, and in which a fixation member is secured. A metal sleeve grips around the outer envelope. The fixation member is welded to the sleeve after the electric element has been brought into a defined position relative to reference points of the lamp cap through shifting, rotating, and/or pivoting of the lamp vessel.
A metal sleeve around the envelope in the cited patent document forms an alternative for the construction in which the sleeve directly grips the relevant neck-shaped portion. This construction may be used in a lamp having an outer envelope, such as also known from, for example, EP-A 0 570 068 (to which U.S. Pat. No. 5,461,277 corresponds), EP-A 0 581 354 (PHN 14.128), and EP-A 0 579 326 (U.S. Pat. No. 5,412,275), as well as in a lamp without outer envelope. The latter type of lamp is also known from, for example, U.S. Pat. No. 5,216,319), U.S. Pat. No. 5,378,958, and EP-A 0 579 313 (U.S. Pat. No. 5,527,199).
In the lamp of the kind mentioned in the opening paragraph according to Patent Application EP 94 201 416.8 (U.S. Pat. No. 5,677,589), a pinch is provided in the outer envelope adjoining the narrowed portion therein, on which pinch a clamping member bears by which the lamp cap holds the lamp vessel.
The Patent Application EP 94 20 13 18.6 (U.S. Pat. No. 5,646,471) describes a lamp of the kind mentioned in the opening paragraph in which a coating is provided on the outer envelope with which the occurrence of parasitic light in a beam formed by a reflector can be counteracted. A coating having favorable properties for that purpose as regards durability and high light absorption is described in the Patent Application EP 94 20 32 76.4 (PHN 15.094) of earlier date.
The Patent Application EP 94 20 37 50.8 (U.S. Pat. No. 5,619,102) describes a lamp of the kind mentioned in the opening paragraph in which a clamping member is provided around the outer envelope, narrowing towards the relevant neck-shaped portion, while a fixation member of the lamp cap grips the clamping member at its narrow portion.
The Patent Application EP 94 20 35 54.4 (U.S. Pat. No. 5,541,471) describes a lamp of the kind mentioned in the opening paragraph where the outer envelope is formed from UV-absorbing quartz glass doped with cerium, titanium, europium, and aluminium.
A discharge lamp of the kind mentioned in the opening paragraph is also known from U.S. Pat. No. 5,109,181 and EP-A 0 576 071 (U.S. Pat. No. 5,497,049). The lamp has a mercury pressure of approximately 200 bar during operation and accordingly emits light having a continuous spectrum. Lamp power has values of up to 150 W and electrode spacing is approximately 1 to 2 mm. The lamp vessel has a small internal diameter of up to approximately 5 mm and a small internal length of up to approximately 8 mm. The lamp cap of the lamp may be made of insulating material and have a contact for each current supply conductor. Alternatively, the lamp may be permanently accommodated in a reflector such as known, for example, from EP-A 0 595 412 (U.S. Pat. No. 5,506,464) or as described in Patent Application EP 94 20 09 60.6 (U.S. Pat. No. 5,568,967) may be used, for example, for projection purposes. The lamp may in that case have a lamp cap of insulating material or of metal which supports a contact to which a current supply conductor of the lamp is connected. The other current supply conductor may be connected to a contact supported by the reflector.
Because of the comparatively small dimensions of the lamp vessel, it can scarcely be avoided that a location of the lamp vessel where an exhaust tube was tipped is in the path of the radiated light, causing deflection of that light, which may be detrimental in the application of the lamp. In addition, solid or liquid ingredients of the lamp vessel filling may condense in that location, whereby they would be removed from the discharge. It is accordingly desirable to provide the filling ingredients in the lamp vessel before the second seal is made, so that a separate exhaust tube and its fused tip are avoided.
The comparatively small dimensions of the lamp vessel also render it necessary for the lamp vessel to be freed of impurities before it is sealed. The impurities could reduce lamp life or be deposited in the light path on the lamp vessel and cause stray light then.
Quartz glass, i.e. glass having a SiO.sub.2 content of at least 98% by weight within the scope of the invention, has a very low linear coefficient of thermal expansion of approximately 10*10.sup.-7. Metals which can be used as current supply conductors under the high thermal loads to which they are exposed in electric lamps have much higher coefficients, for example W approximately 45*10.sup.-7 and Mo approximately 54*10.sup.-7. This means that a wire made from one of these metals embedded in quartz glass at the melting or softening temperature of quartz glass will contract more strongly upon cooling down than does the surrounding glass. The wire then will detach itself from the inside of the glass. The glass will not close around the wire in a vacuumtight manner.
Nevertheless, vacuumtight seals of quartz glass around such metals can be obtained provided the metal has the shape of a foil with sharp edges, also called knife edges or feather edges, because the quartz glass can adapt itself snugly to the shape of the foil, and provided the quartz glass adheres to the foil. To achieve the strength which a current supply conductor must have within and outside the lamp vessel, current supply conductors consisting of a metal foil to which a wire is connected at one end as an internal wire conductor and a wire at the other end as an external current wire, for example by welding, are practically always used in electric lamps having quartz glass lamp vessels. Mo is often used herein as the metal of the foil because this metal has a comparatively high ductility. Current supply conductors often consist of W (internal current wire), Mo (foil), and Mo (external wire).
The expansion of materials implies that no vacuumtight embracing of a current supply conductor by the quartz glass is possible where the conductor is embedded in the quartz glass of a lamp vessel in as far as the internal current wire extends from the cavity of the lamp vessel to on the relevant end portion of the foil, and in as far as the external current wire extends from outside the quartz glass to on the relevant end portion of the foil. The internal and external current conductors have a capillary space around them over these lengths.
In the manufacture of the electric lamp of the kind mentioned in the opening paragraph, the current supply conductors and the electric element are positioned in the lamp vessel under manufacture, and the lamp vessel may be flushed with an inert gas such as, for example, argon, possibly while being heated, so as to drive out impurities. Flushing with inert gas provides a much more effective and fast cleaning than evacuation of the lamp vessel after it has been sealed at one end. Repeated evacuation and flushing with inert gas also has a low effectivity.
Flushing with inert gas, for example in that the second neck-shaped portion is held in a valve from which a flow of inert gas issues, is also useful for preventing oxidation caused by the penetration of air and/or combustion gases from burners when the first neck-shaped portion is locally heated for making a seal therein.
Once the quartz glass has softened sufficiently, it must be pressed against the relevant current supply conductor by means of pinching blocks acting against the pressure of the inert gas which is still flowing, so as to make the first seal. Then the filling is introduced into the lamp vessel and held fixed therein through cooling of the lamp vessel adjacent the first seal, while the free end of the second neck-shaped portion is held by the valve and is kept sealed off from the surroundings. The lamp vessel is then sealed up by locally heating the quartz glass of the second neck-shaped portion and a seal is made over the relevant current supply conductor. Owing to the absence of a gas flow and owing to the comparatively low pressure in the lamp vessel caused by cooling of the lamp vessel, the glass of the second neck-shaped portion, sucked on by the under pressure in the lamp vessel, collapses onto the current supply conductor, thus forming the second seal. The seal may be shaped afterwards, if so desired, with pinching blocks.
It was found that the second seal formed by collapsing and possibly shaped by pinching is more resistant to the changing temperature and pressure conditions of the lamp during and after operation than the first pinched seal.
U.S. Pat. No. 4,389,201 describes the manufacture of a similar electric discharge lamp. The lamp vessel under manufacture is here flushed with argon while the current supply conductors with the electrodes and the solid and liquid ingredients of the filling are being introduced. The free end of the first neck-shaped portion is subsequently heated until the glass has softened to the point where it collapses and closes the relevant portion. The argon flow, however, must be interrupted for this, so that impurities such as air or combustion gases from the burner can penetrate the lamp vessel. When the free end has been closed, argon is admitted into the lamp vessel to a pressure of less than 1 bar, and the two seals are made by causing the glass of the neck-shaped portions to collapse. It is a disadvantage of this method that cleaning of the lamp vessel is interrupted and its effect wiped out in order to close the free end. It is also a disadvantage that an additional operation is to be carried out for closing this free end. A further disadvantage is that ingredients of the lamp filling, for example hygroscopic salts, are in open communication with the surroundings of the lamp during this additional operation and may accordingly themselves absorb impurities.